Negative or positive thinking really boils down to what we choose to dwell on. When I was doing a year long martial arts course during which I knew I would be sorely tempted to give up, whenever the hypothetical thought arose 'what about giving up', I squashed it immediately, knowing that even a little time spent dwelling there would be highly dangerous.
When the stakes are high we find we can police our thoughts pretty easily. But when life itself is a bit easy, with downtime and time spent accomodating the plans of others it may not appear so necessary. Then suddenly you find that you are dwelling long and often on very negative areas of thought. The injunction to think positive doesn't work really. It is too general. Rather it is more functionally effective to focus on the parts of anything that are positive and light filled. Become a guided missile seeking the positive in anything and just blasting past and never alighting on 'what is wrong with the world'. Most terrible are all the problems of nations and tribes- they will suck you in for years. If you can find something in all that mess that is positive go for it but be warned. Aim your positive seeking attennae at all the sources of positive 'nutrition' out there. How about feeding on, and giving away, laughter and light?